1. Field of the Invention
The invention relates generally to circular shift codes, and more particularly, to codes for providing indications of position within spiral-written servo information.
2. Background Information
It is known to provide servo information to a data storage disk through a spiral write operation. The spiral write operation reduces the time required to write the servo information on the disk, from what would otherwise be a circumferential write operation per track. As part of the spiral write operation, a servo track writer moves a write head in a controlled manner, that is, at a constant velocity or along a velocity profile, from an outer diameter of the disk to an inner diameter of the disk (or vice versa) as the disk spins, such that a given spiral crosses each circular track only once. As needed, multiple spirals may be written.
For a spiral write operation, the write head is continuously enabled while the head is moved over an entire stroke from the outer diameter to the inner diameter, or vice versa. In systems of interest, synch marks consisting of a small bit pattern are written at fixed time intervals within the spiral, to aid in transducer position determination. Since the servo track writer moves the write head at a known velocity and the time intervals between the synch marks are known, the distances between respective synch marks along the spiral can be determined. Each synch mark corresponds to a unique radius, and a count of the synch marks from a reference radius provides radial position information. The system must, however, maintain an accurate count as the transducer moves over the disk, that is, servos on the spirals, in order to provide accurate radial position information.
The spiral may, for example, be written as a single frequency pattern, with the synch marks embedded in the spiral as phase shifts. A reader reads from the portion of the spiral that crosses a given circular track and the associated read envelope is rhomboidal in shape. The peak of the read envelope occurs when the center of the spiral write pattern coincides with the magnetic center of the reader. Precise position information can be determined based on where the central-most synch mark occurs in the read envelope relative to the centroid of the read envelope. The synch marks work well for precise position determination, however, for radial position determination they require that the count be maintained, as described above.